Adequate resources to obtain information and to evaluate drugs are essential. Medical information sources include three categories: primary, secondary, and tertiary. Addresses and websites are provided in annex 4.1.

• The primary literature includes journal articles and unpublished studies. These may be obtained from journals and services (electronic or otherwise) that provide the entire articles. An original article contains the most complete information about a subject because readers have access to all the data and study methods and therefore can draw their own conclusions. The disadvantages are that readers must have sufficient time to read and evaluate the article, the skills to evaluate it and compare its information with that in other articles.

• The secondary literature includes indexing and abstracting services that provide abbreviated reviews of articles. Such literature is usually published in newsletters, CD-ROM databases and online services, for example the Cochrane Library. The main advantage of such information sources is that the information is accessible and easy to read. A disadvantage may be the length of time between publication of the original data and its republication in a newsletter or abstracting service.

• The tertiary literature consists of published textbooks. These are usually very good sources of information if reputable and current sources are used. The advantage of textbooks is that one can read and assimilate the information in a relatively short time, since all the information is in one volume. The disadvantages are the lack of access to the original information sources, bias introduced by the writers of the text, and information becoming outdated because of long delays in publishing a text.

• Information from pharmaceutical companies should be used with caution, since such information is biased in favour of positive results in order to promote sales. These materials are usually tailored to the various health professions. They may take the form of scientific articles in professional journals, symposia proceedings, news reports or pamphlets distributed by drug representatives. See also section 7.4.2

• The Internet is a rapidly expanding source of drug information. Although pharmacists or physicians in many parts of the world may not have Internet access, it is a resource that should be used if at all possible. However, it is best to use only those sources that have been recommended by reputable sources and to verify the source of information available on the Internet (WHO 1999b), as the quality of drug information from other sources may be either good or poor.